Pigments and dyes have typically been used to impart color to objects that are otherwise uncolored. Pigments generate color by two main pathways: 1) absorption; and 2) interference.
Traditional pigments and dyes typically generate color by absorption of certain wavelengths of light, while reflecting or transmitting other wavelengths. The reflected or transmitted light corresponds to the color seen when observing a surface that has been coated with or contains organic pigments. For example, a typical “blue” pigment will absorb yellow and red light but it will reflect or transmit blue light, as well as a fraction of violet and green light.
Pearlescent pigments do not typically generate color by absorption and instead, light interference is used for coloration. In these pigments, a reflective substrate is coated by one or more layers of an inorganic material with a different refractive index. As light interacts with the pigment, some light is reflected and some light is transmitted. Depending on the thickness of the inorganic coating, different wavelengths can be reflected or transmitted. This is due to constructive or destructive interference, which can amplify or negate the transmitted or reflected light. Generally, the reflected light does not match the transmitted light, leading to different colors depending on the viewing mode. These colors are known as complimentary colors.
Another interesting feature of pearlescent pigments is that they are also known to reflect near-infrared (NIR) and/or infrared light depending on the materials of composition and the layer structure of the pigment. To these ends, there have been a number of applications where pearlescent pigments have been used to reduce the transmission of near-infrared light, thus producing a thermal insulation effect.
Coatings or plastic sheeting that contain light-reflecting particles for infrared reflection and are transparent to visible light may be used in greenhouses. Additionally, pearlescent pigments may be used as a way to reflect NIR light for the heating and cooling of buildings.
More recently, pigments that have both NIR reflectance and exhibit control over the visible light have been described. Many of these pigments are comprised exclusively of pearlescent pigments that have a color due to interference.
In this technology, we have found that combining traditional pigments and/or dyes with pearlescent pigments in a transparent coating system and/or film provides a means to achieve high degree of control over the visible light spectrum while still having a high degree of NIR reflectance. These coating systems are well suited for use in many applications including decorative, insulating and greenhouse coatings and films where a high degree of spectral control in addition to NIR reflectance is desired.
Due to the cost-sensitive nature of greenhouse maintenance, inexpensive and effective solutions are required. Two major issues important to greenhouses are to: 1) increase the efficiency of crop growth; and 2) increase the efficiency of heating and cooling in greenhouses through the different growing seasons. One way to increase the efficiency of crop growth is to only allow specific wavelengths of light into a greenhouse. The specific spectrum required is the PAR spectrum, with focus on the RFR region, which can affect different crops. Likewise, the efficiency of heating and cooling can be increased by incorporating NIR-reflective materials into greenhouse windows or window coatings that help mitigate heat loss or transmission in the greenhouse.
Plants typically require a specific visible light spectrum, known as photosynthetically active radiation (PAR), in order to achieve the most efficient growth. The PAR region is located in the visible light region of the electromagnetic spectrum between the wavelengths of 400-750 nm. Plants are most sensitive to red and orange light (590-750 nm), followed by blue light (400-495), while they are least sensitive to green and yellow light (495-590 nm). Accordingly, the plants grow most efficiently in an environment where the fraction of green and yellow light is reduced. Furthermore, there is some seasonal variation to the PAR spectrum depending on the growth stage and type of the plants.
Greenhouse owners would like a coating that selectively filters green light while transmitting blue and yellow light. An additional requirement of such a coating is that the transmission of near-infrared (NIR) light is optimized so as to allow better temperature control in the winter and summer months. A further requirement of the coating is to allow some ultraviolet light transmission so that pollinators can navigate the greenhouses. Another important parameter in plant growth is something called the “Red to Far Red” (RFR) ratio. This is a measure of the red light at 660 nm to the far-red light at 730 nm and determines whether a plant will grow tall or wide. An ideal greenhouse coating: 1) Will transmit a well-defined PAR spectrum with fine-tuned RFR ratios; and 2) Exert control over the NIR transmission. These combined issues require an NIR-reflecting pigment with a high degree of spectral fidelity in the visible region.
The pigments used in greenhouses must have high light and weatherfastness. Certain absorption pigments typically have better performance in this regard. The absorption pigments used in the transparent coating system and/or film of the present technology come from all types of pigment classes and include dyes, organic pigments, organic pigment derivatives and/or inorganic pigments.
Absorption pigments generate color through the absorption of light. When light interacts with these pigments, certain wavelengths are absorbed and the remainder of the light is reflected and/or transmitted. The reflected light spectrum is similar to the transmitted light spectrum and corresponds to the observed color. Moreover, the reflected an transmitted light spectra have well defined peaks and sharp transitions, allowing for bright and vibrant colors.
Pearlescent pigments are pigments that generate color by the interference of light. In pearlescent pigments one or more layers of a high-refractive index material envelops a transparent, plate-shaped substrate. Depending on the thickness of the layers in the pearlescent pigment, different wavelengths of light are constructively and destructively interfered, leading to color generation. Often, the reflected light is different than the transmitted light, and pearlescent pigments can have different spectra when viewing them in transmission or reflection modes.
The use of pearlescent pigments as a NIR reflecting pigment is well known. Pearlescent pigments have been known to reflect certain wavelengths of visible light while transmitting others, allowing spectral tunability in the visible region for both reflected and transmitted light. These properties allow pearlescent pigments to be used in applications such as heat reflectors in architectural paints or as light filters in greenhouse coatings. Although, pearlescent pigments reflect and transmit visible light, the individual spectra are inadequate. This is because the interference peaks are typically shallow and/or broad leading to faint coloration. For many applications such as decorative and greenhouse coatings, sharper transitions and narrower peaks are desired in the visible region.
Organic and inorganic pigments (i.e. absorption pigments) have also been used as materials in light-filtering applications. These pigment systems are used when more spectral control is needed in the visible region and have been used in similar applications as pearlescent pigments: heat reflecting architectural paints or as light filters in greenhouse coatings. Although the use of these pigments provides excellent control of the visible light spectrum, these pigments do not have appreciable NIR reflectance. Furthermore, they do not have the transparency of pearlescent pigments which is required for certain applications.
Much of the technology directed to greenhouse design focuses on only one parameter. For example, the use of absorption pigments for greenhouse coatings provides excellent control over the PAR region of the spectrum but these pigments are not active in the NIR and thus no improvement in heating/cooling efficiency is maintained. Conversely, when greenhouses use pearlescent pigments, NIR reflectance is enhanced, but fine control over the visible light is not achieved. Moreover, most of the pearlescent pigments used in greenhouse systems, like the Merck Solarflair®, are comprised of multiple layers of inorganic oxides and can be difficult and/or expensive to make.
In the prior art, either all absorption or all interference pigment systems are used to tune absorbance, transmittance and reflectance spectra from the visible to the NIR regions of light. The present technology describes the use of absorption pigments in combination with pearlescent pigments. The combination of organic pigment and pearlescent pigment makes control of the transmission and reflection spectra in the visible and the NIR regions much easier. Thus, the combination of one or more pearlescent pigments and one or more absorption pigments is blended into a transparent coating system and/or film to aid in the spectral control of both visible and NIR light.